JP5113082B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to an exhaust gas purifying catalyst that removes harmful components contained in exhaust gas from internal combustion engines such as automobiles and motorcycles.

  In recent years, carbon dioxide emitted from internal combustion engines such as automobiles has become a problem from the viewpoint of protecting the global environment. As a solution, a lean burn engine with a small amount of fuel is promising.

By the way, what contains NOx occlusion materials, such as an alkali metal, is used with a noble metal as a catalyst which purifies NOx contained in exhaust gas (refer patent document 1). This catalyst stores NOx in the exhaust gas in the NOx storage material, and removes the stored NOx by the action of the noble metal when the fuel concentration in the exhaust gas becomes rich.
Japanese Patent Laid-Open No. 2002-361094

  However, since the exhaust gas of the lean burn engine mentioned above has a low fuel concentration in the exhaust gas, the conventional catalyst cannot sufficiently purify NOx.

  Moreover, since the alkali metal which is a NOx occlusion material has a low melting point, it moves and dissolves in the catalyst base material when the temperature of the catalyst becomes high, resulting in a decrease in the performance of the catalyst.

  The present invention has been made in view of the above points. For example, an exhaust gas purifying catalyst that has high NOx purification performance even with respect to exhaust gas from a lean burn engine such as a gasoline engine or a diesel engine, and does not deteriorate catalyst performance even at high temperatures. The purpose is to provide.

The present invention
A catalyst substrate;
A catalyst coating layer formed on the catalyst substrate and comprising (a) Rh, (b) Pt, (c) an alkali metal or alkaline earth metal element, and (d) a ZrTi composite oxide or CeZr composite oxide ; ,
An exhaust gas purifying catalyst having
The catalyst coat layer is
A layer structure comprising an inner layer in which 70% by weight or more of (a) is present and an outer layer in which 70% by weight or more of (b) is present ;
The inner layer, viewed contains an element selected from the group consisting of alkaline earth metal elements and rare earth elements, a composite oxide of zirconia,
The gist of (d) is an exhaust gas purifying catalyst which is contained in at least the outer layer .

The exhaust gas purifying catalyst of the present invention has a high temperature (for example, 350 to 450 ° C.) by having an inner layer containing a complex oxide of an element selected from the group consisting of alkaline earth metal elements and rare earth elements and zirconia. (C) The alkali metal or alkaline earth metal element can be prevented from moving and dissolving in the catalyst substrate. Further, the exhaust gas purifying catalyst of the present invention can exhibit high NOx purifying performance even with respect to the exhaust gas of a lean burn engine.

Further, in the present invention, the (a) at least 70 wt% of Rh is present in the inner layer, wherein (b) more than 70 wt% of Pt is present in the outer layer. Thus, by separating (a) Rh and (b) Pt, it is possible to suppress a decrease in the activity of the noble metal and further improve the NOx purification performance.

The (c) alkali metal or alkaline earth metal element and the (d) ZrTi composite oxide or CeZr composite oxide can be included in at least the outer layer. By doing so, the NOx purification performance is further improved.

A alkaline earth metal elements wherein the inner layer comprises (e.g., Ca, Mg) and rare earth elements (e.g., Y, Ce, La, etc.) composite oxide of an element and zirconia selected from the group consisting of the higher temperature conditions Even under the above, (c) the alkali metal or alkaline earth metal element can be prevented from moving and dissolving in the catalyst substrate . In this composite oxide, the amount of components other than zirconia (for example, Ca) is 1.0 to 8% by weight, preferably 1 to 4% by weight in terms of oxide. By being 1.0% by weight or more, the effect of preventing (c) alkali metal or alkaline earth metal element from moving / dissolving in the catalyst base is even higher, and is 8% by weight or less (preferably 7%). (Weight% or less), the characteristics of zirconia are not lost, and the preparation of the inner layer does not become difficult.

  Note that the composite oxide in this specification means not only a mixture of a plurality of oxides but also a solution in which other oxides are dissolved in the oxide structure by heat treatment or the like. The method of forming the latter composite oxide is not limited to the heat treatment, and other known methods can be used.

  Moreover, the position of the said inner layer should just be a catalyst base material side rather than an outer layer, but the position adjacent to a catalyst base material is preferable.

  The exhaust gas purifying catalyst of the present invention may contain a noble metal (for example, Pd, Ir, etc.) other than Rh and Pt. The noble metals other than Rh and Pt may be contained in one of the inner layer and the outer layer, or both the inner layer and the outer layer. In particular, Pd may be contained in either one of the inner layer and the outer layer, or both the inner layer and the outer layer, but considering the viewpoint of suppressing sintering of Pt, the layer containing Pt It is preferable to add to the same layer. Moreover, the catalyst coat layer may be provided with layers other than the inner layer and the outer layer, for example, between the inner layer and the outer layer, further outside the outer layer.

  Examples of the alkali metal element in the component (c) include Li, Na, and K. Examples of the alkaline earth metal element in the component (c) include Ba and Sr. The component (c) may be present in both the inner layer and the outer layer, or may be present only in the outer layer.

  The catalyst base is not particularly limited as long as it is usually used for an exhaust gas purification catalyst, and examples thereof include a honeycomb type, a corrugated type, and a monolith honeycomb type. The catalyst base material may be any material as long as it has fire resistance. For example, use a monolithic structure type made of ceramic such as cordierite or a metal such as ferritic stainless steel. Can do.

It is explanatory drawing showing the structure of the catalyst for exhaust gas purification. It is explanatory drawing showing the structure of the catalyst for exhaust gas purification. It is explanatory drawing showing the structure of the catalyst for exhaust gas purification.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 ... Exhaust gas purification catalyst 3 ... Base material 5, 105 ... Inner layer 7, 107 ... Outer layer 9 ... Catalyst coating layer

  The present invention will be described based on examples.

  a) First, the structure of the exhaust gas-purifying catalyst 1 of the first embodiment will be described with reference to FIG.

The exhaust gas-purifying catalyst 1 is formed by forming an inner layer 5 on the surface of a substrate (catalyst substrate) 3 and further forming an outer layer 7 thereon. The inner layer 5 and the outer layer 7 function as a catalyst coat layer. The substrate 3 is a monolith honeycomb carrier having a volume of 1.0 L and a cell number of 400 cells / in ² , and the inner layer 5 and the outer layer 7 are formed on the inner surface of the cells of the substrate 3.

  The inner layer 5 has the following composition.

Rh: 0.5 g / L
Composite zirconia oxide added with 2% by weight of Ca in terms of oxide: 50 g / L
Ba
Li
K
The outer layer 7 has the following composition.

Pt: 2g / L
Alumina: 100 g / L
ZrTi composite oxide: 100 g / L
CeZr composite oxide 20g / L
Ba
Li
K
In addition, Ba, Li, and K are contained in the whole inner layer 5 and outer layer 7, and the total compounding amount is 0.2 mol / L for Ba, 0.1 mol / L for Li, and 0.15 mol for K. / L. Ba, Li, and K are components that act as nitrogen oxides.

  b) Next, a method for producing the exhaust gas-purifying catalyst 1 of the first embodiment will be described.

First, slurry S1 and slurry S2 were prepared by mixing the following components.
(Slurry S1)
Rh chloride solution: 0.5 g Rh
Composite zirconia oxide added with 2% by weight of Ca in terms of oxide: 50 g
Water: 100g
(Slurry S2)
Alumina: 100g
ZrTi composite oxide: 100 g
CeZr composite oxide: 20 g
Water: 200g
Next, slurry S1 was coated on the entire substrate 3, dried at 250 ° C. for 2 hours or longer, and then fired at 500 ° C. for 1 hour or longer to form the inner layer 5. Further, the slurry S2 was coated on the slurry S1 previously, dried at 250 ° C. for 2 hours or more, and then fired at 500 ° C. for 1 hour or more to form the outer layer 7. Next, it was immersed in a Pt nitrate solution (2 g Pt), and 2 g / L of Pt was supported on the outer layer 7.

Next, it is immersed in an acetic acid solution containing 0.2 mol, 0.1 mol, and 0.15 mol of Ba, K, and Li, respectively, and after the excess aqueous solution is blown off, it is dried at 350 ° C. for 30 minutes or more for exhaust gas purification. Catalyst 1 was completed.
(Reference Example 2)

The configuration of the exhaust gas purifying catalyst 1 of the present reference example 2 is basically the same as that of the first embodiment, but is partially different in the inner layer 5. Below, it demonstrates centering on the difference. The inner layer 5 contains an oxide of only zirconia (hereinafter referred to as “zirconia”) instead of the composite zirconia oxide added with 2% by weight of Ca in terms of oxide.

The manufacturing method of the exhaust gas-purifying catalyst 1 of the present Reference Example 2 is basically the same as that of Example 1, but the slurry S3 was used instead of the slurry S1 as the slurry for forming the inner layer 5. . Slurry S3 was prepared by mixing the following materials.
(Slurry S3)
Rh chloride solution: 0.5 g Rh
Zirconia: 50g
Water: 100g
( Example 3 )

  The configuration of the exhaust gas purifying catalyst 1 of Example 3 is basically the same as that of Example 1 except that the outer layer 7 is not only Pt but also Pt (1.5 g / L) and Pd as noble metals. (0.5 g / L).

The manufacturing method of the exhaust gas-purifying catalyst 1 of Example 3 is basically the same as that of Example 1. However, after forming the outer layer 7, not the Pt nitrate solution but the PtPd nitrate solution (Pt. 5 g, 0.5 g / L with Pd), and Pt 1.5 g / L and Pd 0.5 g / L were supported on the outer layer 7, respectively. Thereafter, the steps of supporting Ba, K, and Li are the same as in the first embodiment.
(Comparative Example 1)
The configuration of the exhaust gas purifying catalyst 101 of Comparative Example 1 is such that a single catalyst coat layer 9 is formed on the surface of a substrate (catalyst substrate) 3 as shown in FIG. The substrate 3 is the same as that of the first embodiment.

  The catalyst coat layer 9 has the following composition.

Alumina: 100 g / L
Zirconia: 50 g / L
ZrTi composite oxide: 100 g / L
CeZr composite oxide: 20 g / L
Rh: 0.5 g / L
Pt: 2g / L
Ba: 0.2 mol / L
Li: 0.1 mol / L
K: 0.15 mol / L
In order to manufacture the exhaust gas-purifying catalyst 101 of Comparative Example 1, first, the following components were mixed and wet-pulverized to prepare slurry S4.
(Slurry S4)
Alumina: 100g
Zirconia: 50g
ZrTi composite oxide: 100 g
CeZr composite oxide: 20 g
Rh: 0.5g
Next, the entire substrate 3 was coated with the slurry S4, dried at 250 ° C. for 2 hours or longer, and then baked at 500 ° C. for 1 hour or longer to form the catalyst coat layer 9.

  Next, the catalyst coat layer 9 was impregnated with 2 g / L of Pt by being immersed in a Pt nitrate solution, and then dried.

Further, Ba, K, and Li were supported on the catalyst coat layer 9 in the same manner as in Example 1 to complete the exhaust gas purification catalyst 101.
(Comparative Example 2)
As shown in FIG. 3, the exhaust gas purifying catalyst 101 of Comparative Example 2 has an inner layer 105 and an outer layer 107 formed on the base material 3. The composition of the inner layer 105 is the same as that of the outer layer of Example 1. The composition of the outer layer 107 is equal to the composition of the inner layer 5 in the first embodiment. That is, in the exhaust gas purifying catalyst 101 of Comparative Example 2, the inner layer and the outer layer are reversed in comparison with Example 1.

  The exhaust gas-purifying catalyst 101 of Comparative Example 2 was manufactured as follows. First, the entire surface of the substrate 3 was coated with the slurry S2, dried at 250 ° C. for 2 hours or longer, and then fired at 500 ° C. for 1 hour or longer to form the inner layer 105. Next, the inner layer 105 was supported with 2 g / L of Pt by immersing in a Pt nitrate solution (2 g of Pt).

Further, the slurry S1 was coated on the slurry S2 previously, dried at 250 ° C. for 2 hours or longer, and then baked at 500 ° C. for 1 hour or longer to form the outer layer 107. Next, Ba, K, and Li were supported on the inner layer 105 and the outer layer 107 in the same manner as in Example 1 to complete the exhaust gas purification catalyst 101.
(Comparative Example 3)
The exhaust gas purifying catalyst 101 of Comparative Example 3 is basically the same as that of Reference Example 2, it differs in part in the inner layer 5 and the outer layer 7. Below, it demonstrates centering on the difference. Rh is not included in the inner layer 5 but is included in the outer layer 7. That is, in Comparative Example 3, both Rh and Pt are included in the outer layer 7.

( Comparative Example 4)
The configuration of the exhaust gas-purifying catalyst 101 of Comparative Example 4 is basically the same as that of Example 1, but is partially different in the inner layer 5. Below, it demonstrates centering on the difference.

  The inner layer 5 has the following composition.

Rh: 0.5 g / L
Alumina: 50 g / L
Ba
Li
K
That is, in this comparative example 4, the inner layer 5 does not contain zirconia oxide, and instead contains alumina.

The manufacturing method of the exhaust gas-purifying catalyst 101 of Comparative Example 4 is basically the same as that of Example 1, but the slurry S7 is used instead of the slurry S1 as the slurry for forming the inner layer 5. . Slurry S7 was prepared by mixing the following components.
(Slurry S7)
Rh chloride solution: 0.5 g Rh
Alumina: 50g
Water: 100g
(Comparative Example 5)
The configuration of the exhaust gas-purifying catalyst 101 of Comparative Example 5 is basically the same as that of Comparative Example 4, but is partially different in the outer layer 7. Below, it demonstrates centering on the difference.

  The outer layer 7 has the following composition.

Pt: 2g / L
Alumina: 220 g / L
Ba
Li
K
That is, in this comparative example 5, the outer layer 7 does not contain a ZrTi composite oxide or a CeZr composite oxide.

The manufacturing method of the exhaust gas-purifying catalyst 101 of Comparative Example 5 is basically the same as that of Comparative Example 4, but the slurry S8 is used as the slurry for forming the outer layer 7 instead of the slurry S2. . Slurry S8 was prepared by mixing the following components.
(Slurry S8)
Alumina: 220g
Water: 500g
(Comparative Example 6)
The configuration of the exhaust gas-purifying catalyst 101 of Comparative Example 6 is basically the same as that of Example 1, but differs in that Pt is disposed in the inner layer 5 and Rh is disposed in the outer layer 7. Below, it demonstrates centering on the difference.

  The inner layer 5 has the following composition.

Pt: 2g / L
Composite zirconia oxide added with 2% by weight of Ca in terms of oxide: 50 g / L
Ba
Li
K
The outer layer 7 has the following composition.

Rh: 0.5 g / L
Alumina: 100 g / L
ZrTi composite oxide: 100 g / L
CeZr composite oxide 20g / L
Ba
Li
K
The manufacturing method of the exhaust gas-purifying catalyst 101 of Comparative Example 6 is basically the same as that of Example 1, but the slurry S9 was used instead of the slurry S1 as the slurry for forming the inner layer 5. . Slurry S9 was prepared by mixing the following components.
(Slurry S9)
Nitric acid Pt solution: 2g of Pt
Composite zirconia oxide added with 2% by weight of Ca in terms of oxide: 50 g
Water: 100g
Further, after the outer layer 7 is formed using a slurry S2, using a Rh chloride solution to supporting Rh in the outer layer 7.

Next, tests performed to confirm the performance of the exhaust gas-purifying catalyst 1 manufactured in each example will be described.
(1) Evaluation of Alkali Metal Remaining Amount in Catalyst Coat Layer (1-1) Test Method First, an exhaust gas purifying catalyst is attached to a 2000 cc gasoline engine, stoichiometric conditions at 750 ° C., and lean conditions at 700 ° C. Deterioration accelerated durability (hereinafter referred to as “endurance”) was repeated 40 times alternately.

  Next, only the catalyst coat layer was scraped from the exhaust gas purifying catalyst, dissolved in an acid, and the amount of alkali metal (K) was measured by chemical analysis (ICP). The measurement was performed on the exhaust gas purifying catalysts of Examples 1 to 3 and Comparative Examples 1 to 6.

Table 1 shows the amount of alkali metal (K) remaining in the catalyst coat layer. The initial amount of the alkali metal that is incorporated into the catalyst coating layer (K) is, in Example 1, 3, are equal in either of Reference Examples 2 and Comparative Examples 1 to 6.

As shown in Table 1, the exhaust gas purifying catalysts of Examples 1 and 3 and Reference Example 2 had a larger amount of residual alkali metal (K) in the catalyst coating layer than Comparative Examples 1, 2, 4, and 5. . This is because in the exhaust gas purifying catalysts of Examples 1 and 3 and Reference Example 2 , zirconia contained in the inner layer 5 prevents (c) migration / solid solution of the alkali metal or alkaline earth metal element to the base material 3. It is thought that it is to do.

In particular, in the exhaust gas purifying catalysts of Examples 1 and 3, the remaining amount of alkali metal (K) in the catalyst coat layer was larger. This is because the zirconia composite oxide added with Ca contained in the inner layer 5 has a higher effect of preventing (c) migration and solid solution of the alkali metal or alkaline earth metal element to the base material 3. It is believed that there is.
(2) Evaluation of catalyst activity (2-1) Test method First, durability was performed on the exhaust gas purifying catalyst in the same manner as in the above "(1-1) Test method". Next, a model gas containing HC, O 2 and NO was passed through the exhaust gas purification catalyst, and the HC purification rate in a lean atmosphere was measured while raising the catalyst temperature at a rate of 100 ° C. to 30 ° C./min. The definition of the HC purification rate is a value represented by (A−B) / A, where A is the HC concentration on the catalyst entrance side and B is the catalyst exit HC concentration. The experiment was performed on Examples 1 and 3, Reference Example 2 , and Comparative Examples 1 and 3 (Pt and Rh are mixed in the same layer).

  The HC purification rate improved as the catalyst temperature increased. For each exhaust gas purification catalyst, the temperature at which the HC purification rate reached 20%, 40%, 50%, 70%, and 90% was determined. The results are shown in Table 2.

As shown in Table 2, the exhaust gas purifying catalysts of Examples 1 and 3 and Reference Example 2 were lower in temperature to reach a predetermined HC purification rate than Comparative Examples 1 and 3. That is, the exhaust gas purifying catalysts of Examples 1 and 3 and Reference Example 2 had higher catalytic activity than Comparative Examples 1 and 3. This is considered to be because in the exhaust gas purifying catalysts of Examples 1 and 3 and Reference Example 2 , the catalytic activity is increased by blending Rh in the inner layer 5 and blending Pt in the outer layer 7.
(3) Evaluation of NOx reduction amount (3-1) Test method First, an exhaust gas purifying catalyst was attached to a 2000 cc gasoline lean burn engine, and a lean atmosphere of A / F = 20 to 28 was obtained. Then, the reduction amount (mg) of NOx was calculated from the difference between the NOx concentration on the catalyst entrance side and the NOx concentration on the exit side at that time. The other conditions during the measurement were as follows.

  The NOx concentration on the outlet side was measured until it reached 80% or more of the concentration on the inlet side.

Engine speed: 1200-3000rpm
Torque: 20 ~ 70N ・ m
Entering gas temperature: 300 ° C, 350 ° C, 400 ° C, 450 ° C, 500 ° C
The experiment was conducted on the exhaust gas purifying catalysts of Examples 1 and 3, Reference Example 2 and Comparative Examples 1 to 6.
The results are shown in Table 3.

As shown in Table 3, the exhaust gas purifying catalysts of Examples 1 and 3 and Reference Example 2 have high NOx purification performance. In particular, when the incoming gas temperature is 450 to 500 ° C., any of Comparative Examples 1 to 6 is used. NOx purification performance was also high. Further, the exhaust gas purifying catalysts of Examples 1 and 3 had significantly higher NOx purification performance than any of Comparative Examples 1 to 6 at all temperatures. From this result, it has been clarified that the exhaust gas purifying catalysts of Examples 1 and 3 have high NOx purification performance even with respect to the exhaust gas of the lean burn engine.

  Needless to say, the present invention is not limited to the above-described embodiments, and can be implemented in various modes without departing from the scope of the present invention.

  For example, in Example 3, 0.5 g of Pd may be included in the inner layer 5 without being included in the outer layer 7. Further, both outer layer 7 and inner layer 5 may contain Pd.

Claims (3)

A catalyst substrate;
A catalyst coating layer formed on the catalyst substrate and comprising (a) Rh, (b) Pt, (c) an alkali metal or alkaline earth metal element, and (d) a ZrTi composite oxide or CeZr composite oxide ; ,
An exhaust gas purifying catalyst having
The catalyst coat layer is
A layer structure comprising an inner layer in which 70% by weight or more of (a) is present and an outer layer in which 70% by weight or more of (b) is present ;
The inner layer, viewed contains an element selected from the group consisting of alkaline earth metal elements and rare earth elements, a composite oxide of zirconia,
Said (d) is contained in at least said outer layer, an exhaust gas purifying catalyst. The exhaust gas-purifying catalyst according to claim 1, wherein (c) is included in at least the outer layer. Further, Pd, said inner layer, and / or exhaust gas purifying catalyst according to claim 1 or 2, characterized in that it comprises in the outer layer.